Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment, as examples. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon.
Some semiconductor devices comprise memory devices that are used to store information. A recent development in semiconductor memory devices is magnetic random access memory (MRAM) devices, in which the spin of electrons is used to indicate the presence of a “1” or “0” as digital information. MRAM devices comprise conductive lines (word-lines and bit-lines) positioned in different directions, e.g., perpendicular to one another in different metal layers. The conductive lines sandwich resistive memory elements that comprise magnetic tunnel junctions (MTJs), which function as magnetic memory cells. MTJs include two ferromagnetic layers separated by a thin insulating tunneling barrier. One ferromagnetic layer is a fixed or pinned layer and the other is a free layer that changes resistive state by a change in magnetic polarity when programmed. Digital information stored in an MTJ is read by detecting the MTJ's resistive state.
One type of MRAM device is a spin-transfer torque switched MRAM (STT-MRAM), in which local magnetization is used to program MTJs by exerting a spin torque on a magnetic moment of the free layer of MTJs.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.